Thermally hardening steel



Patented Dec. 26, 1944 UNITED STATES PATENT OFFICE THERMALLY HARDENINGSTEEL John M. Hodge, Munhall, and Max W. Lightner, Pittsburgh, Pa.

. No Drawing. Application April-20, 1940,

Serial No. 330,798

Claims. (Cl. 148-12) This invention is a method or thermally hardening asteel article having a portion with a higher carbon content than anotherportion. An example of such an article is a steel plate having acarburized surface providing the portion of higher carbon content thananother portion,

the latter being the uncarburized portion of the plate. Other carburizedsteel articles also may content, the austenite of a higher carbon steeltransforming more slowly than the austenite of a lower carbon steel.

with the above in mind, the present invention is generally characterizedby including the heating of an article having a portion with a highercarbon content than another portion, so as to Also, the austenitictrans-' the range of from 100 C. to 400 C. Molybdenum, chromium,vanadium, tungsten, silicon, manganese, .boron/ nickel, tantalum, etc.all have'sthis tendency more or less. In addition .to exerting thiseffect, these elements also function to increase the hardenability ofsteel to which they are added over that of plain carbon steel of similarcarbon content, and for this and other reasons elements of this, typeare frequently used to roduce alloy steels intended for carburization.and hardening.

render'both these portions austenitic, the article then being quenchedto the temperature range.

'of from 100 C. to 400 C. and held within this range untiltransformation of at least'a majority of the austenite of the portion oflower carbon content is effected, whereupon the article is.

quenched to below the defined temperature range prior to transformationof more than minor amounts of the austenite of the higher carbon portionand at a rate producing martensite.

The result of the. above is a steel article having an at least partiallymartensitic portion and another portion predominantly comprisingproducts resulting from austenite transforming in the temperature rangeof from 100 C. to 400 C. Thus, the article will be very hard at oneportion and .,.,will have another portion that is hardened'b'utpossesses the ductility andimpact strength. provided by productsresulting from austenitic transformation within the defined tem:-

perature range- It is to be understood that metal hardened by quenchingto and holding within the defined temperature range, may be recognizedby studying its microstructure since this is an acicular type ascontrasted to amartensitic structure produced by otherhardening methods.

Alloying elements other than carbon also have a decided efiectinretarding the austenitic trans- Since the austenitic transformation rateof plain carbon steel is comparatively rapid, the present inventionfurther includes the addition to the steel of the article previouslydiscussed, while the steel is molten, of one or more alloying elementsother than carbon, of such a character and used in such amounts as tomaterially retard the austenitic transformation rate of the steel in thedefined range as compared to that of plain carbon steel of similarcarbon content. The element or elements used may be any of the broadclass hereinbefore mentioned. When such a steel is carburized to producean article having a portion of higher carbon content than anotherportion,

the higher carbon portion has a slower austenitic transformation ratethan that of the portion of comparatively lower carbon content, and ,thetransformation rates of both portions are considerably retarded ascompared to similar portions. in the case of plain carbon steel.,Therefore, the carrying out of the method hereinbefore described ismade easier in that a greater time is provided for manipulation of thework during the various steps involved.

During the time the steel is held at the temperature range of from 100C. to 400'C., the austenite of the lower carbon portion is continuouslytransforming to the particular products produced within this range. Theportion of higher carbon content during that. time does not harden dueto transformation of its austenite and, therefore, isin-a relativelymalleable and ductile state. Consequently, during the period whileaustenite transformation is going on in the case of the portion of thelower carbon content,

the steel article may be plastically formed as desired, such as bycurving it, so that it will have a predetermined shape at the end of themethod.

Austenitic transformation effected in the stat ed temperature rangeresults in a minimumof distortion during the hardening of the steel, and

the higher carbon portion subsequently hardenedto martensite withconsequent expansion formation rate of steel attemperatures withincannot distort the article being hardened as much as usual, because thisexpansion is resist ed by the steel of the lower carbon portion which iscompletely hardened about the time austenitic transformation of thehigher portion is initiated. In t e case of certain articles such as aplate havi g one carburized surface, it is possible to plastically formthe plate while its lower carbon portion is undergoing austenitictransformation, so as to cause its carburized face to be concave to adegree suflicient to result in subsequent hardening of'this surface to amartensitic condition causing this surface to flatten. Also, the platemay be produced by such forming with its carburized surface convex so asto be placed in compression during flattening of the plate. Usualhardening of carburized plate may result in the carburized face beingconcave so that fiattening places this face in tension so as to involvethe possibility of rupturing it.

In some instances it may be desirable to temper the higher carbonportion to relieve some of the strain resulting from its conversion to amartensitic condition by the second described quenching. This temperingmay be done by heating-the higher carbon, portion to any temperatureless than the .temperature range wherein transformation of the austeniteof the lower carbon portion took place. In-other words, the

article should be tempered only by the use of temperatures less thanfrom\ 100 C. to 400 C., the exact temperature depending on thetemperature where the mentioned transformation of austenite occurred.

Although the principles of this invention are applicable to theproduction of a great many carburized articles, such asdies, gears,tools, shafts, bearings, rolls, etc., a specific example of theinvention as it has been commercially applied to the hardening of lightarmor plate will now be disclosed.

The armor plate is rolled from steel to which alloys are added while itis molten so it contains 25% carbon, 50% manganese, 4.5% nickel and .40%molybdenum, the balance being iron excepting for immaterial amounts ofthose elements normally classed as impurities in the case of the productunder discussion. The resulting plate is then carburized on one surfacein the prior art manner. Tests have shown that at 250 C. it requiresabout two hours to effect austenitic transformation producing thedesired hardness in the case of the low carbon portion of this plate,whereas no austenitic transformation occurs in the case of thecarburizedsurface over a period of five hours. Suchinformation must be developedexperimentally in the case of steels of different chemical compositions,but the desired retardation of the austenitic transformation rate isobtained in the case of a wide range water quench or in the case of thinsections, by

immersing the plate directly into the bath used to hold its temperaturewithin the stated range.

. this transformation flattens the plate.

This bath is composed of either lead or a lead alloy adapted to give thedesired fluidity at the holding temperature used, but .oil or salt bathsmay be used for this purpose. all events to control the temperature ofthe bath to keep it constant, it being preferable to hold thetemperature of the plate at a. constant value within the stated rangesince this provides better control of the method.

During the time the temperature of the plate is being held, theaustenite of the uncarburized portion is transforming to the particularproducts formed in the temperature range used, the austenite of thecarburized surface failing to transform to any great extent during thisperiod because of its higher carbon content and, therefore, its muchslower rate of austenitic trans-' formation. As soon as the austenitictransformation of the uncarburized portion is complete,

tenite of its carburized surface to martensite,

this producing a very hard surface. As just Care is taken in stated, thesecond quenching is carried out after to obtain a properly hardenedsurface. The time period of the first quenching and the time forinitiating the second quenching are easily determined factors.

If the plate is flat at the termination of the period effectingaustenitic transformation of the uncarburized portion, the secondquenching con-' verting the carburized surface to martensite,

causes a consequent expansion of this surface so that it becomes convex.This is contrary to what usually occurs in the case of conventionalhardening methods and is important in that subsequent flattening of theplate throws the hardened surface in compression so that it cannotrupture.

To obtain a flat plate, the plate is plastically formed prior tocompletion of the transformation of the austenite of its uncarburizedportion in such a manner as to make its carburized surface concave.Then, upon the subsequent quenching transforming the austenite of thecarburized surface to martensite, the expansion incidental to In thisway, a flat piece of armor plate is produced which requires little or nosubsequent flattening.

.products recognized as resulting from austenitic transformationpractically entirely within the temperature range of from C. to 400 C.If necessary, the hardened surface is tempered by reheating totemperatures less than'the transformation temperature for-suitableperiods of time. As previously mentioned, transformation products ofaustenite produced in the range stated are microscopically recognizable.Further-.

impact strength than can be obtained by conventional hardening methodsin the case of steel of similar chemical composition.

We claim:

l. A method of, thermally hardening a steelarticle-having a portion witha higher carbon content than another portion, said method includingheating said article to render both said portions austeni-tic, quenchingsaid article tothe temperature range of from 100 C. to 400 C., holdingsaid. article in said range until transformation of at least a majorityofthe austenite of the second,

named portion is eflected and quenching said article to below said rangeprior-to transformation of more than' minor amounts of the austenite ofvthe first named portion and at a rate producing austen'itic,quenchingsaid articleto the tempera- 'ture range of from 100 C. to 4000., holding said article in said range until transformation of at leasta majority of the austenite of the second named portion is effected andquenching said ar-g mation of at leasta majority or the austenite of thesecond named portion is effected and quench ing said article to-belowsaid range prior. to transformation of mor than minor amounts of theaustenite of the first named portion and at a rate producing martensite,said method further ineluding plastically forming said article after thestarting of but prior to complete transformation of the austenite of thesecond named portion.

I 4. A method of thermally hardening a steel article having a portionwith a higher carbon content than another portion, said method includingheating said article to render both said portions austenitic, quenchingsaid article to the temperature range of from 100 C.- to 400 C., holdingsaid article in said range until transformation of at least a majorityof the austenite of the second named portion is efiected and quenchingsaid article to below said range prior to trans- .formationof more' thanminor amounts of the austenite of the first named portion and at a rateproducing martensite, said method further including tempering the firstnamed portion at a temperature less than the temperature where theaustenite of the second named portion occurred.

ticle to below said range prior to transformation; of more than minoramounts of the austenite of the first named portion and at a rateproducing. martensite, said method further including the addition to thesteel of said article while it is molten of at least one alloyingelement other than in such amount as to materially retard.the austenitictransformation rate of said steel as compared to plain carbon steel ofsimilar carbon oon-' v3. A method of thermally hardening a steel 5. Amethod of hardening an allow steel plate with a carburized surface,comprising heating said plate to render it entirelyaustenitic, quenchingsaid plate to the temperature range of from C. to 400 0., holding theplate in said range until austcnitic transformation of its uncarburizedportion is practically complete and quenchin -said convex to a degrepreventing said surface from becomin concave when rendered martensitic,.

article having a portion with a higher carbon 40 said pl tic f min beingdone after the startcontent than-another portion, said method in-,cluding heating said article to render both said portions austenitic,quenching said article to the temperature range of from 100 C.'to 400'0.,

"holding said article-in said range until transfor- 0 'ing of but priorto complete transformation of the austenite of the uncarbujrized portionof said plate.

-. JOHN M. HODGE.

- MAX W. LIGHTNER.

